Sure but an 18 year old Rooney scored an amazing hattrick plus an assist in his debut, and 18 year old Ronaldo has had an electrifying performance in his debut too. Just saying, Messi wasn’t as good/established as Rooney and Ronaldo in straight comparison when he was only 18-19, although Rooney and Ronaldo was a little older and more experienced at that time (Ronaldo was 20, Rooney was 19 with few years under their belt when Messi was 18). Messi did turn into a top player when he was 20, but even so he was not as good as Ronaldo yet, who was main contender for Ballon D’or at that time. He did become best in world when he was 22.
Mbappe has been best young player in the world since he was 18 to 22. I would say:
At 18-19, Mbappe was better (Mbappe won WC, impress in CL, more proven, while Messi is more talented but not quite as established).
At 20-21, more or less at same level (both are regarded as one of best, top 3-5 in the world),
At 22, Messi was clearly much better (Messi being the best and far better than others except Ronaldo) Mbappe was still regarded as top 3-5)